This invention relates to a methodology for bonding together two microfabrication substrates.
Microelectromechanical systems are devices which are manufactured using lithographic fabrication processes originally developed for producing semiconductor electronic devices. Because the manufacturing processes are lithographic, MEMS devices may be made in very small sizes. MEMS techniques have been used to manufacture a wide variety of transducers and actuators, such as accelerometers and electrostatic cantilevers.
MEMS devices are often movable, they may be enclosed in a rigid structure, or device cavity formed between two wafers, so that their small, delicate structures are protected from shock, vibration, contamination or atmospheric conditions. Many such devices also require an evacuated environment for proper functioning, so that these device cavities may need to be hermetically sealed after evacuation. Thus, the device cavity may be formed between two wafers which are bonded using a hermetic adhesive.
Deposition techniques for the thin layers used in semiconductor and MEMS devices often leave gases incorporated in the layers during deposition. These devices may then be encapsulated in the evacuated cavity for proper functioning. However, the gases incorporated in the films may escape from the layers during the devices' lifetimes, raising the pressure in the evacuated cavities. Therefore, depending on the degree of vacuum needed, a gettering material may also be enclosed in the device cavity for continuous absorption of contaminant gases.
Accordingly, many designs include such a getter material, which is typically a reactive, metal layer, whose purpose is to absorb these gases by oxidation, in order to maintain the vacuum levels within the package. Because of the reactive nature of these materials, they also tend to oxidize spontaneously at the surface, forming an oxide layer that must be removed in order to activate the getter. Activation of the getter may require exposure to high temperatures, temperatures consistent with bonding using glass frit, as described below.
Devices which use or manipulate electromagnetic radiation, such as emitters, reflectors, absorbers, gratings, and the like, may require encapsulation in an optically transmissive device cavity to function effectively. Glass wafers would provide such a cavity. However, a hermetic seal around a glass cavity typically requires a glass frit adhesive, which may require processing temperatures in excess of 400 C to melt and fuse the frit. Although these temperatures may also be adequate to activate the getter, they may also exceed the temperatures that can be withstood by many of the thin metal layers used to create the optical device. Thus, encapsulation of an optical device in a transparent device cavity which is hermetically sealed has been an elusive goal.
Anodic bonding of a glass substrate to a silicon substrate is known, wherein voltage and heat are applied between the glass wafer and the silicon wafer. The voltage applied promotes the growth of the oxide layer between the silicon and the glass, which adheres the materials together. However, this method requires one of the wafers be a silicon wafer, which, of course, is not transmissive to most portions of the electromagnetic spectrum, including the visible portion.
Accordingly, the packaging of optical devices in a hermetic glass cavity remains an unresolved problem.